Try heading for the coast. Yachtie types often do their own maintenance and servicing on such things and a big chandlery on a coastal marina usually sells a selection of refill / service kits for lifejackets and liferafts.

If you really value this piece of kit, send it to a service agent and get it fully serviced. It's not very expensive - you will be glad when you really need the raft. Those who service their own without training are insane. Would you pack your own emergency parachute without training (if at all)?

Always used Ocean Safety in Southampton for servicing lifejackets and raft when I sailed a lot.

If you need a replacement cylinder, the raft NEEDS a service - you either inflated it, of the cylinder is so old it corroded away - what else has corroded that you can't see?

OTOH, seeing the lifejackets that the comercial airlines supply, it is clear that they don't believe they could ever be used. Even in calm weather, most people would have a problem keeping one of those jackets in the correct position to keep their head out of the water.

Wot No Engines, yes I have had the life raft professionally checked out & repacked. No problems. It just needs a CO2 bottle, which I will get calibrated properly. But I do not have the right bottle ......